Such an optical element is disposed on the optical path of the laser light, which corresponds to each of the plurality of types of recording media, in order to share the objective lens, the optical pickup or the like among the plurality of types of recording media which have mutually different optical characteristics, such as a distance between a substrate surface and a recording layer, and a recording density. As one example of the optical element, there is proposed an optical element which realizes compatibility of the objective lens among the plurality of types of recording media by using diffraction (e.g. refer to a patent document 1).
In the optical pickup, diffraction orbicular zones are provided, and a luminous flux is flared on the outer side of a predetermined numerical aperture in a use condition on the side that the numerical aperture is smaller. Therefore, it is considered that a beam diameter is not narrowed down too much, and that the recording of the information and the reproducing of the information can be performed with respect to the plurality of types of recording media which mutually differ in thickness.
Patent document 1: Japanese Patent Application Laying Open NO. 2001-235676